1. Field of the Invention
The present invention relates to an actuator device for optical deflector, on which an optical deflector for deflecting and scanning a light beam such as a laser beam is mounted.
2. Description of the Related Art
Recently, as one form of an image display device, a projection display has been proposed in which a light beam from a light source is deflected by an optical deflector and is projected onto a screen on which an image is displayed. For the optical deflector, for example, an optical deflector in which a mirror, a piezoelectric actuator and other mechanism elements are integrally formed on a semiconductor substrate has been proposed as a MEMS (micro electro mechanical systems) device using semiconductor processes or micro machine technology (see Patent Document 1).
In this optical deflector, one end of a piezoelectric actuator is linked to and supported by a frame part (support), and a torque, which is generated by the piezoelectric actuator, is transmitted to a torsion bar (elastic beam) that is connected to other end of the piezoelectric actuator, thereby rotationally driving a mirror attached to the end of the torsion bar. Advantages of such an optical deflector include its small size, simple structure, and a large driving force that can be provided.
For the aforementioned projection display, a technology has been proposed which uses a semiconductor laser or other laser beam sources as a light source, which, compared to a lamp, has a longer life as a light source, enables higher efficiency of energy use, and realizes higher purity of three primary colors of light. However, when using a laser beam source as a light source, there is a problem in that speckle noise (granular interference pattern) is generated, resulting in the deterioration of the quality of an image.
Speckle noise is a phenomenon that occurs when a coherent light having a constant phase from a laser beam source is scattered by a random phase plane (object plane), resulting in an interference of a disturbed wave front from adjacent regions of the object plane on an observation plane. This phenomenon appears on the observation plane as a granular intensity distribution. According to the projection display using a laser beam source, the observer is likely to recognize the deterioration of the quality of images when such speckle noise occurs between a screen as an object plane and an observer's eye (retina) as an observation plane. Therefore, various technologies have been proposed to reduce speckle noise (see Patent Documents 2 and 3, for example).
In the image display device described in Patent Document 2, speckle noise is reduced by a polarization distribution converting method, which spatially changes a polarization status of an incident laser beam. Specifically, this image display device displays an image using light which is emitted from a light source and is modulated by a spatial light modulator, and thereby converts the spatial polarization distribution of light emitted from the light source by using the polarization distribution converting means so that the polarization direction of incident lights entering adjacent pixels of the spatial light modulator is orthogonal to each other. Since orthogonal polarized lights do not interfere with each other, speckle noise in the displayed image, which is caused by the interference between the regions corresponding to adjacent pixels of the spatial light modulator, can be suppressed.
Further, in the image display device described in Patent Document 3, the polarization state of an incident laser beam is not changed; instead, an optical component which generates a very small difference in an optical path (to the extent that the image is not influenced; that is, the optical path differences are distributed to the extent that diffraction angles are small enough) is disposed, and by vibrating or rotating the optical component using a drive means so that the distribution of the optical path differences varies over time, generated speckle patterns are combined and averaged, thereby reducing speckle noise that can be observable on the screen.    Patent Document 1: Laid-open Japanese Patent Application No. 2005-148459    Patent Document 2: Laid-open Japanese Patent Application No. 2002-62582    Patent Document 3: Laid-open Japanese Patent Application No. 2004-138669
However, according to the devices described in Patent Documents 2 and 3, an addition of an optical element and/or a drive mechanism to the optical system is required, separately from an optical deflector, to reduce speckle noise. In other words, according to the technique described in Patent Document 2, a polarization distribution conversion means is required, and a laser beam diameter also needs to be expanded beyond the size of the polarization element of the polarization distribution conversion means, thereby requiring an additional lens or enlarging the size of the optical system.
Furthermore, according to the technique described in Patent Document 3, an optical component to change the optical path length and also a drive mechanism to vibrate or rotate the optical component need to be installed. Thus, in the technology described in Patent Documents 2 and 3, problems such as an increase of the number of components, an increase of size as a system, an increase of power consumption, an increase of noise, a complicated optical alignment, and so forth have been found.